Wood-resin composite products, such as plywood, laminated veneer lumber (LVL), oriented strand lumber (OSL), oriented strand board (OSB), waferboard, particleboard, medium density fiber board, hardboard and the like, traditionally have been made by combining a binder resin with wood components (wood pieces) to form a stack or mat which is then consolidated in a hot platen press to cure the binder resin.
In some products, such as plywood and laminated veneer lumber (LVL), an adhesive binder mixture is used in which an aqueous phenol-formaldehyde resole resin liquid constitutes the major ingredient, but that ingredient is supplemented with additional components such as additional caustic, amylaceous extenders such as wheat flour, proteinaceous extenders such as soya flour, fillers such as nutshell flours, tree bark flours and clays, viscosity control agents and the like. Extenders are components which have some inherent adhesive characteristics of theft own and thus can be considered supplemental adhesives in the adhesive binder mixture. Fillers are components which, on the other hand, are not significantly adhesive, but which improve the adhesive binder mixture's working properties, performance, strength or the like.
In preparing other products, such as the manufacture of OSB and waferboard, the phenol-formaldehyde resole resin substantially constitutes the sole adhesive ingredient for the adhesive binder formulation; fillers and extenders are not typically used.
In some cases, the phenol-formaldehyde resin may be supplied in the form of a powder, rather than as an aqueous liquid. Manufacturing OSB and waferboard are applications where phenol-formaldehyde resin powders, in particular, find a sizeable amount of use. The powdered phenol-formaldehyde resins are thought to have a number of advantages over liquid resins in such applications, such as: (a) simple and less expensive equipment often can be used for handling and blending the resin with the wood furnish; (b) powdered resins are easy to blend onto wood wafers and strands, and (c) powdered resins generally have a longer storage life so inventory control is easier to manage.
Historically, such phenol-formaldehyde resin powders are prepared principally by spray-drying a liquid phenol-formaldehyde precursor resin made under alkaline conditions (i.e., a resole resin) in the presence of a caustic, i.e., sodium hydroxide, catalyst. Such resins are fast curing and provide wood composites of high strength.
U.S. Pat. No. 5,217,665 describes making waferboard using both a liquid phenol formaldehyde resin and a powdered phenol formaldehyde resin. According to this patent the liquid and solid resin components are applied separately to the wood wafers. For example, in one embodiment, liquid phenol-formaldehyde resin is applied first to the surface of the wafers followed by an application of powdered phenol-formaldehyde resin, a layup of the so-treated wafers then is formed and the layup of wafers is pressed at an elevated temperature and pressure to consolidate the layup into a board and cure the phenol formaldehyde adhesive. When the liquid and solid resins are used in combination, the solid resin constitutes at least 25% by weight of the total applied phenolic resin.
The present invention is based on the discovery that the addition of a small amount of powdered, e.g., spray-dried, phenol-formaldehyde resole resin into an aqueous phenol-formaldehyde resole resin liquid used an adhesive binder for making wood composites improves production capacity, improves bond quality and expands the operating conditions for making acceptable composite products.